Throughout the development of wireless communication networks, which also include mobile and ad-hoc communication networks, the issue of efficient distribution of radio resources to the users using the radio link has been the subject of different optimization schemes. The parameters relevant in such an optimization scheme are user throughput, delay time, and fairness of distribution of radio resources. Usually, the different optimization schemes aim at maximizing one of these parameters, since they are in conflict. The signaling performed in distributing radio resources to users in a wireless communication network is usually controlled by the MAC (Media Access Control) layer, which is responsible for administrating the access to the radio link among the different users.
Two commonly used traditional UL MAC (Uplink Media Access Control) protocol principles are scheduled access and contention based access to the radio channel. The benefit of using a contention based UL MAC protocol is that any user can start to transmit as soon as there is data to send. This property gives contention based UL MAC protocol an advantage of having low delays when the load is low (and hence the risk of transmissions from two or more users colliding is low). One important shortcoming of contention based MAC protocols are collisions, i.e. when two or more users in a cell or coverage area start transmitting data to the same user simultaneously. Examples of contention-based protocols are Aloha, Slotted Aloha, CSMA (Carrier Sense Multiple-Access), CSMA-CD (Carries Sense Multiple Access with Collision Detection), S-MAC (Sensor Media Access Control), T-MAC (Timeout Media Access Control).
However, when the load is high, a scheduled MAC protocol, such as TDMA (Time Division Multiple Access), TRAMA (Traffic-Adaptive Medium Access) or FLAMA (Flow-aware Medium Access Framework), performs better both in terms of delay, system throughput and collision probability. An UL MAC protocol often makes use of both contention based and scheduled resources. Typically, the users may transmit scheduling request messages on a contention based channel and then they are assigned a scheduled resource for the data transmission.
One such MAC protocol is suggested in Dahlman, E. et al “A Framework for Future Radio Access”, aimed at distributing radio resources among users in a WINNER wireless communication network, where several users share an uplink channel where transmission may be either contention-based or scheduled. Users may transmit resource requests on a contention-based channel while the base station may respond by assigning the corresponding users resources for future data transmissions on a scheduled channel. Users may also piggyback scheduling requests onto ongoing data transmissions on the uplink scheduled channel.
Furthermore, U.S. patent application no. US2006/0050742 describes a wireless communication system where a super-frame structure in the time domain, that consists of contention based transmission periods (time slots) and contention-free transmission periods is suggested. During the contention-free transmission periods, the users transmit on scheduled channels, while they compete during the contention based periods.
One other contention and scheduling based MAC protocol is described in the U.S. patent to Wright et. al. no. U.S. Pat. No. 6,240,083, where a base station periodically sends out a control message on a broadcast channel to all mobile devices on the radio link. Such a control message includes the type and size of a transmission window for the uplink channel and the identifiers of the mobile devices the transmission window is allocated to. The MAC protocol disclosed in U.S. Pat. No. 6,240,083 is however very CDMA-specific.
Usually, when two users collide on a contention based channel the base station may only be able to decode the strongest signal correctly. But the weaker signal will typically not be decoded and the message will have to be retransmitted again after some (typically random) time duration. If the signals colliding have about equal received power there is a high probability that both signals are lost in the collision. But since both users retransmit their messages again after some (random) waiting times the chance that the same users will collide again can be made low. This is for example described in the two patent documents US2006/0050742 and U.S. Pat. No. 6,240,083
The MAC protocols described above, however, have one problem in common. In the event that a user is located far away from the base station the signal will always be received with low power. If the load in the system is high then there is a high probability that the cell edge user will almost always collide with the transmission of some other cell center user that is received with higher power. Even if some kind of (open loop or closed loop) power control is deployed to balance the received powers from nearby and far away users there will always be users that have reached their power limit and therefore can not increase their transmit power any more. In such power limited scenarios the message from the cell edge user will have a hard time reaching the base station when the load is high, since it will always lose in the contention with the other users closer to the base station. This puts an unnecessary limit on the system coverage.
The object of the present invention is therefore to resolve at least some of the problems with known technology.